Is volume the most important muscle mass variable?

Chris Beardsley is a biomechanics researcher. He is also the editor of Strength and Conditioning Research. He is based in Loughborough, Leicestershire.

If you want to add more lean muscle mass in less time, then you need to crank up the volume. I don’t mean on the speakers on the gym stereo, but the amount of volume you lift when you train.
Volume is defined by researchers as the number of sets and repetitions performed per muscle group in a single workout, and it is one of the most important training variables to consider for positive physiological adaptations

Key training variables include exercise selection, exercise order, and tempo, but the factors that sports scientists have explored most in long-term studies include: relative load (the percentage of one-repetition maximum [1RM]); training frequency (the number of times per week you train); the range-of-motion (ROM) used in an exercise; the duration of your rest periods between sets; and of course, volume.

Volume studies have mainly explored the effect of volume on hypertrophy by comparing training programmes containing multiple sets with those containing just single sets. In fact, they tend to be even more specific and usually directly compare three-set programmes with one-set programmes.
When enough studies have been performed investigating a single training variable, such as volume, other mathematically-minded researchers can gather up all of the results from each individual study and combine them using a special statistical technique called meta-analysis.

This technique is very powerful because it allows researchers to determine the overall trend across several studies for that particular variable. Luckily for us, enough studies have been performed comparing different training volumes for a meta-analysis to be feasible. Indeed, one has been performed.

Meta-analysis on volume
In 2009, a well-known researcher and statistician called James Krieger carried out a meta-analysis comparing the effect of single and multiple sets on gains in muscular size. On the basis of the studies available at the time, he reported that multiple sets (mostly three sets) are associated with 40% greater hypertrophy-related effect sizes than single sets, in both trained and untrained individuals (effect sizes are statistics that give us an indicator of the size of the difference between two training programmes).
We can’t be completely certain solely on the basis of this meta-analysis that multiple sets are better for gains in muscular size than single sets because of inherent limitations in grouping the data from different studies.

Volume in the dock
Indeed, several researchers criticised this meta-analysis, mainly because part of the power of any meta-analysis lies in the similarity of the included studies. Meta-analyses with very similar studies tend to be quite powerful and they can give us a good indication that the resulting trend is sound. Meta-analyses including studies with big differences between individual studies in the key elements (such as the length of the training program, or the training status of the individuals) can be prone to error.

My friend and colleague, James Fisher, criticised Krieger’s meta-analysis, noting that there were large differences in the training status of the individuals of the studies included, as well as differences in respect of the relative loads, which ranged from six to eight repetitions through to 15 repetitions, among other issues.

I value Fisher’s opinion highly, but I think perhaps this time he is being overly sceptical. After all, while there are some differences between the studies included, it seems pretty clear which way the trend is going, and accounting for the differences doesn’t alter the end result.

Fortunately, since the date of the meta-analysis performed by Krieger, there have been at least four studies that have explored the effects of volume on hypertrophy, which have helped clear up the issue once and for all.

Volume revisited
One study, conducted by Bottaro et al., compared the effects of different resistance training volumes on increases in the muscular size of the quadriceps and biceps brachii in untrained young men. The researchers randomly assigned the subjects into two groups who trained two days per week for 12 weeks.

One group performed three sets of knee extensions and one set of biceps curls each workout. Conversely, the other group did one set of knee extensions and three sets of biceps curls each workout. The researchers found that muscle thickness of the biceps brachii increased significantly for both groups while changes in the quadriceps were not significant for either group. They also found that although there were no significant differences between the groups, there was a non-significant trend for the three-set arm group to display a greater increase for the biceps brachii (7.2% for the three-set arm group compared to 5.9% for the one-set arm group).

Another study, Radaelli et al., compared the effects of different resistance training volumes on increases in muscle thickness of the biceps brachii and quadriceps in 20 healthy, older women. The subjects were randomly assigned into two groups who trained two times per week for 13 weeks. The groups performed either one-set or three-sets of biceps curls and knee extensions. The researchers found that muscle thickness measurements of the biceps brachii and quadriceps increased similarly in both groups. Additionally, there was a non-significant trend for the total quadriceps muscle thickness to increase by more in the three-set group than in the one-set group of 14.3% (± 4.1%) versus 8.6% (± 2.0%).

20 weeks vs 13 weeks
The third study, also by Radaelli and colleagues, then examined the impact of twice per week training for 20 weeks, using 20 healthy, older women using the same two exercises and training protocols. They found that muscle thickness measurements of the biceps brachii and quadriceps increased in both groups. However, in this longer, 20-week study they reported that gains in quadriceps muscle thickness were significantly greater in the three-set group than in the one-set group.

The fourth study, Sooneste et al, compared the effects of different resistance training volumes on increases in the muscular size of both arms of the same subject in a crossover-like design with different training volumes (one or three sets) in eight sedentary, untrained young Japanese men. The subjects trained their biceps brachii two days per week for 12 weeks, using a seated dumbbell preacher curl with 80% of their one-repetition maximum (1RM). The researchers found that the three-set training programme led to significantly greater increases in muscular cross-sectional area than the one-set training programme.

Out of the four studies performed since the meta-analysis by Krieger, two reported a significant benefit of using three sets rather than one set and the remaining two studies found a non-significant benefit. Thus, these results are consistent with Krieger’s earlier meta-analysis and using multiple sets is definitely better than using a single set.

Practical implications
Based on the research, it is clear that training with multiple sets to achieve a higher volume of training leads to greater hypertrophy, irrespective of training status and age. Practically speaking, this means we can use greater training volumes to progress in the same way as we try to progress towards heavier weights. In fact, if you’re just aiming for gains in muscular size, then increasing the volume you can handle might even be a better target for you than increasing the weight you are lifting. But, just like increasing the load you are using, you don’t need to make big jumps in volume, as this can impair recovery.

Recovery is always an important factor when altering any training variable to enhance gains in muscular size. Consequently, the huge volumes of training that you might see professional bodybuilders using are likely not optimal for you unless you are a very experienced lifter. The workload will be too great, you won’t be able to recover in time for your next workout, and you probably find yourself overreaching and maybe even overtraining.

For example, if you’ve been following a minimalist HIT-style programme of one set per muscle group for the last couple of months, then moving up to two sets per muscle group might be more than enough to elicit gains. Indeed, you might even want to limit the increase in sets to just a few of your main exercises. Just like increasing the load of the weights you lift, small increases in volume over time will add up to big gains in the end.

Why is volume effective for hypertrophy?
Exactly why volume is so important for hypertrophy is difficult to say. It could be because the muscle is under mechanical tension for a longer period of time (perhaps also leading to greater metabolic stress), it could be because more work is done, or it could be because the muscle is stretched more frequently (possibly causing more muscular damage). In reality, it could be any combination of these factors. It’s too early to tell.

Fortunately, there are no prizes awarded for guessing which of the biological mechanisms are responsible for each of the rules when it comes to maximising hypertrophy. Rather, the biggest wins come from knowing what the rules are in the first place. Knowing the rules is what helps us create the best resistance-training programmes for maximising muscular size. And here we can identify our second rule: greater volumes lead to the greatest gains in hypertrophy and should be preferred where recovery is possible.